It is sometimes desirable to know the internal temperature of integrated circuits. The integrated circuit temperature can be used to improve circuit operation by compensating signals that change from nominal operation when the chip temperature changes.
For example, in a non-volatile memory device such as a NAND flash memory, programming and reading memory cell data requires various voltages for programming the cell to a desired threshold voltage and then applying that threshold voltage to the cell to determine if the cell turns on and conducts. If the cell does not turn on, it has not been programmed to the desired threshold.
During operation of the integrated circuit, the temperature varies both due to ambient temperature, as well as to the electrical operation of the integrated circuit, causing a temperature increase. The temperature change can cause a change in the operating characteristics of the memory cell. For example, a threshold voltage of 1V at room temperature may turn into a threshold voltage of 900 mV as the chip temperature increases. The change in voltage levels can have an impact on reading, programming, and verifying operations that are expecting a certain voltage.
Temperature change in a dynamic random access memory (DRAM) device can also have an impact on memory operation. A DRAM requires periodic refresh cycles in order to maintain the integrity of the data stored in the memory. The temperature of the memory device affects the frequency at which the memory device needs to be refreshed. As the device heats up, the cells lose their ability to hold a charge due to current leakage. Therefore, the warmer the device the more often it has to be refreshed.
Designers typically take into account the worst case requirement for refreshing a DRAM and design the memory to refresh at a fixed rate consistent with the maximum operating temperature of the part. However, the faster refresh rate is not required when the device is operating at a cooler temperature, thus wasting power. It would therefore be beneficial to be able to determine the memory device's internal temperature in order to adjust the refresh rate in response to changing temperature.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for temperature sensing in an integrated circuit.